fix date and month args and add some traits

[rafaqz]

This PR:

• fixes the docs for recent changes
• adds missing interface methods, like Tuple of dates always working
• removes defaults for month/date. They are just confusing and hard to work with. month and date just have to be specified.
• add some traits for minor differences between types, not yet exported.

[codecov[bot]]

Codecov Report

Merging #39 (f92195c) into master (06aed5c) will decrease coverage by 1.50%. The diff coverage is 23.33%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   73.41%   71.91%   -1.51%     
==========================================
  Files          15       15              
  Lines         395      413      +18     
==========================================
+ Hits          290      297       +7     
- Misses        105      116      +11     

Impacted Files	Coverage Δ
src/alwb/alwb.jl	81.25% <0.00%> (-11.61%)	:arrow_down:
src/awap/awap.jl	77.41% <0.00%> (-5.34%)	:arrow_down:
src/chelsa/bioclim.jl	100.00% <ø> (ø)
src/earthenv/habitatheterogeneity.jl	93.10% <ø> (ø)
src/earthenv/landcover.jl	100.00% <ø> (ø)
src/worldclim/bioclim.jl	100.00% <ø> (ø)
src/worldclim/climate.jl	100.00% <ø> (ø)
src/worldclim/weather.jl	17.64% <0.00%> (-0.54%)	:arrow_down:
src/shared.jl	62.50% <37.50%> (-0.36%)	:arrow_down:
src/chelsa/future.jl	62.31% <40.00%> (+0.19%)	:arrow_up:
... and 1 more

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